Big refactoring of file storage handling

- Moved pillar.api.utils.{gcs,storage} to pillar.api.file_storage_backends
- Implemented GCS and local storage using abstract Bucket and Blob classes
- Removed file processing from the Blob class, and kept it in the
  file_storage/__init__.py class. That way storage and processing are
  kept separate.

pillar/api/utils/imaging.py

@@ -5,6 +5,7 @@ from PIL import Image
 from flask import current_app
 
 
+# TODO: refactor to use pathlib.Path and f-strings.
 def generate_local_thumbnails(name_base, src):
     """Given a source image, use Pillow to generate thumbnails according to the
     application settings.